"Oval" center

[ed4copies]

In reading another post, the subject of tightening the mandrel has come up. This has started a conversation, but I would like to hear MANY opinions on this one. I frequently find my mandrel "whipping" in the center, creating an out-of-round condition. Conventional wisdom has been: the nut is too tight, loosen it. I have tried this, with varying success. But why SHOULD it work? The mandrel goes into the headstock on a Morse taper, so it comes out straight. Tightening the nut puts pressure on the length of the mandrel and, if the bushings are all "square", the pressure should be "stretching" the mandrel-exactly straight.

Another possiblility is that the tailstock is not lined up 100% correctly-but the "tailstock end" of the pen is NOT out of round.

Hope this will start a conversation that all can learn from. Or, maybe I'm the only one who ever had this problem (on all 3 of my lathes???)

 

[DCBluesman]

The discussion is going on at this post http://www.penturners.org/forum/topic.asp?TOPIC_ID=5727&whichpage=1

 

[dougle40]

Ed ,
Check to see that nothing is inside your MT , this could be the cause also .

 

[Mac In Oak Ridge]

This is going to be interesting. I have been reading the same stuff for a year or so. The part about making sure your tail stock is "exactly" centered on the head stock. The thing about putting a pointed center in both and bringing them together, and all the contortions to get them so they line up exactly.

While it would be nice if they did, it doesn't make any difference in turning things between centers. The old saw about the shortest distance between two points come to mind.

If you have a piece of wood on a live center that is dead on with the head stock it will turn perfectly round if you are a skilled turner. If you have the same piece of wood on a live center that is 1/4" off from the head stock center it will turn perfectly round if you are a skilled turner.


Now if you face off the end of that piece of wood (the one in the offset set up) then the face would not be at right angles to the centerline of the piece. If the 1/4" offset was up and down instead of front to back then it would face off perfectly at 90° to the center line of the piece of wood.

 

[Old Griz]

Your right Mac it will turn perfectly round if you are skilled turner and your tailstock is off my 1/4", but the piece will be tapered. That is what people who do eccentric turning to... they mount the piece off center to get the eccentricity... which is the same as having the tailstock off center...
You should always set up your lathe so that the tailstock and headstock are in the same plane...
As for having out of round pens, the problem is usually that the mandrel is bowed.. this does not come from overtightening the brass nut, but by putting too much pressure on the end of the mandrel by overtightening the live center at the tailstock causing the mandrel to bow... I tighten the brass nut just enough to keep the blanks from turning and tighten the live center until it is just spinning freely...

 

[ed4copies]

The tailstock possibility is very helpful-at least it makes sense to me.
The crud in the morse taper I have already considered and cleaned.
My only other thought relates to the tolerance between the bushings and the mandrel.
Recently, I have not run into the problem, but I am using almost all new mandrels (an attempt to solve). And, I am not using PSI, which I believe is a couple thousandths smaller than everyone else.
Still interested in ideas.

 

[RLKnecht]

The lathe will turn the pieces PERFECTLY ROUND with good technique and sharp tools. The overtightening of the tail stock will cause the center of the piece to be off (due to the mandrel bowing), not the piece to be oblong or out of round. Look at a turned piece that you messed up on before (come on all of us have 'em). Note it is perfectly round, but the center is not in the correct place.
The moral of the story, only tighten the tail stock and the nut as tight as needed to stop things from slipping and nothing more. You can usually loosen the nut after knocking off the corners to minimize the bowing due to loose bushings. That is not as big of a deal as the tail stock, but every little bit helps.
Rick

 

[lkorn]

True PSI IS a little smaller but so is the internal diameter of their bushings. I had to polish off the coating from a Woodcraft mandrel in order to accomodate the PSI bushings. Then I bought a new Berea mandrel for theirs and Woodcraft bushings.

Originally posted by ed4copies
<br /> And, I am not using PSI, which I believe is a couple thousandths smaller than everyone else.

 

[ed4copies]

Until recently, I did not label mandrels (I have collected about a dozen). As I became more aware of the misplaced centers (you're right, they really are NOT oval), I have reorganized. So, I may very well have been using Berea bushings with PSI mandrels from time to time.

THAT problem is now less likely to occur. NEVER say NEVER.

 

[JimGo]

Ed,
Tom said it best, but one subtle thing that may contribute to a misalignment of the headstock and tailstocks is if the feet of the lathe are not on the same plane too. That is, if the stand on which you've placed the lathe isn't quite flat, then the weight of the lathe may actually cause it to twist a bit, resulting in misalignment. What I plan on doing when I finally get some time that I'm willing to spend in my shop but NOT turning, is to make sure the headstock and tailstock are properly aligned, and that the mandrel is "true", as per Russ Fairfield's instructions (http://www.woodturnerruss.com/Pen12b.html ).

Rick, you're right, when the blank is close to its final size, it is essentially round, but the center is off, which makes for the feeling I discussed in the other link (the one Lou pointed to, above).

 

[GregD]

I have to agree with Old Griz on this. Too much pressure is being placed on the end of the mandrel by the tailstock causing the mandrel to bow.
Here is what I do. I run my tail stock up to the mandrel and lock it in place, then turn on the lathe and tighten the live center until it is just turning and lock it in place. You don't need a lot of pressure. It's just there to hold the end of the mandrel in place.

 

[Rudy Vey]

I think a lot of this out of round has to do with too much pressure during the turning process and subsequently flexing of the mandrel! Try the following (best done if you have a collet chuck and can shorten your mandrel appropriately). Turn with both blank halves installed and then do another pen (same wood and pen type) with the mandrel shortened and only one blank piece at a time. There is a difference: the shorter mandrel does not flex as much as the long one and the out-of-round will be much less. A good help in avoiding too much out of round is: having very sharp tools and do not try to hog-off too much at a time. Also, some woods are more prone to this than others since wood is not a very homogeneous mass like metals.

Sometimes you will also get whistling sounds and some vibrations, this normally happens always in the middle of the mandrel, never towards the end. I have started to turn certain pen models in their halves only, like the Baron and the Gents, since they have only bushings for 7 mm (A) mandrel and this flexes more than the thicker "B" mandrel. Another way to avoid showing the oval is to put the side which would show off the "ovality (sp)" more, towards the headstock (for example for the am. flat tops this would be center area, the clip end is not so important).

Overtightening of the tail stock surely will bow the mandrel, thus will flex more and easier. I do not believe that over-tightening the nut (if the blanks are squared) will do so. I have done some testing with spring washers (lock washers) between the nut and the bushing to avoid a very tight nut. If the blanks are not square I can imagine some slip towards being out of round. A lot of things combined: no sharp tools, there fore too much pressure, too tight on the tail stock, no square blanks etc: this all will add up in out of round.

I had discussed this out-of-round issue with Jim H once and he pointed me in direction of the mandrel flex.

 

[opfoto]

Everyone,
I have turned a couple of (oval in the middle) Cigars recently. I too have been quietly wondering why this has happened. I did the finger test before placing the blanks on the mandrel. I use the Woodcraft adjustable mandrel. I also have 2 replacement mandrels, I believe these were all made by berea for woodcraft. I will make sure that I do not over tighten the mandrels or the tail stock. Hopefully the suggestions offered here will help. I will let you know how it turns out!

 

[swm6500]

I tend to agree that it is the pressure from the tailstock more than anything.

 

[opfoto]

Just an Update....
I just finished 2 SL's. (will post pics later) Realizing the SL is a much smaller pen, so the problem may be harder to feel. The "oval" center problem seemed to have lessened. I became anal about checking the knurled knob. I checked it after blanks were round. Then a couple of times more while turning to style sizes. I also checked them while I sanded them(I did say anal). On a couple of times I noted the knurled nut did seem to tighten. (strange) I will do a couple of cigars and try to imitate the same level of analness(?) and see if there is a difference. Will update again then.

 

[JimGo]

Marc, I noticed that in some cases, where there wasn't much vibration in the mandrel (which is, after all, the goal), and especially on the pens with larger diameter bushings, the knurled nut will ride the bushing. If the blank closest to the bushing has some "catches" or otherwise slides with respect to the mandrel, I think this can cause the knurled nut to twist, and actually tighten. Ironically, if there is vibration in the mandrel, I've actually seen the opposite happen; the knurled nut will wind up down near the tailstock no matter how hard (within reason) I tighten it.

 

[Fred in NC]

I said this before, but another possible cause...

The ends of the barrel are not perfectly square to the tube. When you tighten the nut the mandrel will bow...

It is like inserting a wedge... think about it

 

[jdavis]

Agree with Fred about squarness.

 

[opfoto]

Fred,
Thanks for the tip. I'll definitely give that a try as well. I normally give it a shot with the trimmer. But I do use the trimmer til I see the entire circle all the way around on my blanks. I'll spend a bit more time on that part as well. Thanks again!!!

 

[Fred in NC]

You can check for bow in the mandrel before you turn. That helps before it is too late.

I measure from the tool rest to the center bushing. I hold a small ruler on top of the tool rest, and bring it to contact the bushing. Then rotate the spindle by hand.

Also, with the spindle rotating. you can hold the tip of a screwdriver, etc., lightly against the top of the center bushing. It will show any vibration which means the bushing is not rotating on center. This is good also for testing the mandrel without any bushings.

 

[scubaman]

There are a couple of misconceptions in this thread, I think. First, the point that started the thread - "when you tighten the nut you just stretch the mandrel straight". This might be true in theory, but in reality often things are not PERFECTLY square. This could be the square shoulder on the blank, or debris caught between bushing and blank. When you tighten the nut, you are putting an uneven load on, that can bow the mandrel. It doesn't take much! You then turn at high speed and you get the "whipping"

The second point about offset tailstock. (I am not aware of the previous discussion.) On many (all?) wood lathes (definitely all mini-lathes) the tailstock is not adjustable. But the point I want to make is that alignment DOES matter some. And in the metal-turning world, you don't turn a taper by JUST offsetting the tailstock. You also need to put the workpiece between centers, and drive it appropriately using a 'dog'). You can't put steel in a chuck, offset the tailstock, and expect this to work. So, here, with a mandrel, you are NOT turning between centers! Consider the following: The tailstock is set back from the operator side by 1/4". Now put a mandrel in the headstock. It will miss the live center. Bend it back to force it. What does the mandrel look like? Probably a smooth bow, with the outside of the bow towards the operator. Now draw a (imaginary) line on the mandrel. Turn the headstock half a turn. Now that line is away from the operator, on the inside of the bow! And this is what happen constantly as you turn - you're really distorting the mandrel! No way this will result in a usable pen.

The notion of a skilled turner still being able to turn a perfect cylinder: possible. Definitely true without the mandrel, even I could probably turn that. But, everything changes with a piece of steel getting flexed around inside. OK, assume your mandrel flexes so nicely that you do end up with perfectly cylindrical barrels. That's on the outside. Now stop the lathe and think about the mandrel describing a bow inside the perfect cylinder. At the ends, it's probably reasonably concentric. But in the middle it definitely is NOT! Non-concentric, offcenter, some people call it oval...

Of course I exaggerate, 1/4" is not realistic, and there are some effects that actually counteract the bowing, like that very mandrel nut. Bottom line is don't go there... the better the alignment is, the better you're off.

 

[Rudy Vey]

Hi Rich, nice to see you here, too!!

 

[JimGo]

Rich (and others), do you think it would help if, instead of using a live center with a point, you were to use something that could close around the mandrel (kind of like a Jacobs or Collet chuck, but one that also rotated freely)? Granted, you'd still have the alignment issue, but at least with something like what I'm (hopefully) describing, you could be sure that the tailstock wasn't adding unnecessary pressure along the mandrel's axis of rotation. I guess the cup center mentioned above would be similar, although the Jacobs or Collet-style chucks would probably give the mandrel more strength since the mandrel would actually extend into the chuck.

 

[scubaman]

Jim, not sure this is as easily engineered as it sounds ;-) I don't think there is anything fundamentally wrong with using a live center. Here's what you do: After you turn the pen, loosen the tailstock and make sure it is just supporting the mandrel. Loosen the mandrel nut and fingertighten it. The reason this works is that you get more catches before the blank is rounded. And when you do, the blank rotates a little and takes the mandrelnut with it thus tightening it. Once you're past that rough stuff get your setup the way it should be. Try it, if you have a concentricity problem. This probably fixes 90% of all 'out of round' problems

With a fixed mandrel, the tailstock pressure should not change, but what the heck - this is the time to make sure

What happpens with an adjustable mandrel (including the Berea drill driver or any mandrel that is not screwed into a morse taper, like one held in a drill chuck) is that when you get a catch, the mandrel nut tightens as I mentioned. So it exerts pressure onto the bushings and barrels and eventually against the adjustable holder, and it will pull the mandrel shaft out from that holder. Which puts the extra pressure on the tailstock.

 

[Darley]

Nice to see you on the IAP forum Richard and thanks for your help and tutoring

Serge

 

[JimGo]

Thanks Rich! I never thought about going back and loosening the knurled nut after roughing down the blanks, but it makes a lot of sense.

Thanks again!

 

[Daniel]

I learned the Nut will tighten while roughing otuthe blanks the hard way. when I had two or three hard catches then couldn't get the nut off. actually had to mount the mandrel in a vise and use a pair of pliers to loosen it. on a brass nut. well it was pretty much the end of the nut after that. not from the outside. the threads where shot. actually had a groove in the bushing side of the nut. now I just quickly loosen and retighten the nut after any hard catch, and just before I start the final turning.

 

[arehrlich]

Daniel,

Good ideas. I will begin to examine the tightness of the brass nut more often now. I've noticed that it tightens and never really paid attention to why, or what the consequences might be.

Thanks for the advice.

Alan

 

[ed4copies]

Thanks to ALL!!

Turned a group of pens this week, watching for the "squareness" and adjusting after roughing out. As I have said before, I do NOT have a light hand while taking off the square edges. But, thanks to this thread, I now readjust the blanks after turning to round and the problems have been eliminated.

I appreciate all your help and have several dozen past efforts that will now be "collector's items"-off-center center bands. []

 

[DCBluesman]

Originally posted by ed4copies
<br />As I have said before, I do NOT have a light hand while taking off the square edges.

Ed, if you have a disk/belt sander, you might try knocking off the edges of your blanks. It takes a little stress off your roughing.

 

[dougle40]

After having just receiving my new mandrel lately , the one thing that I've noticed is that the brass knurled nut has a tendancy to tighten itself (which brass will do) while I'm turning , this can cause a bow in the centre of the mandrel . For this reason I've gone back to using a standard nut rather than the brass one .

 

[scubaman]

Originally posted by dougle40
<br />After having just receiving my new mandrel lately , the one thing that I've noticed is that the brass knurled nut has a tendancy to tighten itself (which brass will do) while I'm turning , this can cause a bow in the centre of the mandrel . For this reason I've gone back to using a standard nut rather than the brass one .

I'm curious about that, DOug. Do you think it's simply the fact that the nut is larger, and it tightens because of the increased angular momentum as the lathe speeds up? What mandrel are you using, and is it threaded 1/4-28 or 1/4-20? (Most mandrels have 1/4-28, I believe Berea is the only exception perhaps). I sort of doubt that this effect could be enough to warp a mandrel - but I won't argue with success []. Maybe you can do the following test: chuck up a pair of blanks, tighten the nut as you normally would. Then lock the spindle, grab the blank on the right and twist it away from you. Do you feel it tightening? Any difference between the 2? And when you use the hex nut, do you tighten it by hand or using a wrench? I never quite understood why people do the latter, but I have seen accomplished pen turners do it. When I find a blank slipping initially, I usually tighten the nut using the blank.